Using Tasers as a threat --- comply immediately or things could go very badly.

Anthony Pirone took the witness stand this morning at the murder trial of a former colleague on the BART police force, explaining to jurors how and why he detained train rider Oscar Grant minutes before the colleague shot and killed him.

Pirone, a critical witness for both the prosecution and defense in the trial of Johannes Mehserle, said Grant and four of his friends fit a vague description of suspects in a fight aboard a train that had pulled into Fruitvale Station in Oakland early Jan. 1, 2009.

A police dispatcher, relaying information the train operator had gotten from a passenger, had said the suspects were black men in black clothing on the lead car. Grant was African American.

Pirone said he began cursing at the men almost immediately after spotting them on the platform, and threatened to shock them with a Taser as a means of "intimidation" to gain compliance.

Pirone said he had ordered three men to sit against a wall. Grant and a second man initially tried to hide inside a train car, Pirone said, but he found them and pulled them out.

He said he had found Grant walking from one car to another using the interior doors, then had directed the laser light of his Taser at Grant through a window. He said he had ordered Grant off, then had told him to "get the f- off the train."

Grant soon came out, Pirone said, and cooperated as he was escorted to the wall, though he complained and swore as he went.

This afternoon, Pirone is expected to testify about why he ultimately decided to arrest Grant for allegedly resisting officers, and about what happened in the moments before Mehserle shot Grant while trying to handcuff him. Grant was unarmed and on his chest.

Witnesses at the trial have said Pirone's profanity and aggressiveness in detaining the men angered other BART riders. Grant's relatives believe Pirone escalated the situation, and that racial profiling was a factor in his initial detention of the five men.

But this is the real kicker:

Pirone could influence the jury's conclusions on two major issues in the case: whether Grant was resisting Mehserle and whether, as the defense contends, Mehserle accidentally shot Grant while intending to shock him with a Taser.